Depends on how much of a traditionalist you are. Not having a vent creates a more formal look and that's why it's more traditional. Side vents, on the other hand, are more practical as they prevent the back of the jacket from riding up when you put your hands in your pockets. A single rear vent was designed for horseback riding so not only is it informal but it also has a tendency to open up and reveal your derriere when you put your hands in your pockets. (Some might say it's bad form to place one's hands in one's pockets but the gentleman in my avatar would disagree!)

Don't the pants need a single ribbon as well? Without one, a black-tie outfit just looks... odd (I've also worn full military dress uniform #1, blue pants with a thick red-stripe... without the red stripe it just wouldn't work)